Ink jet printing head and industrial plotter which is equipped with it

ABSTRACT

Ink jet printing head and replacement of bevel sticks in industrial tracers by an ink jet printing head provided with a set of deflection plates (3, 4, 5) so arranged and polarized that the electric field of deflection of droplets is orientable according to an angle which may vary from 0° to 180°. The applications cover particularly the field of industrial tracing techniques.

FIELD OF THE INVENTION

The invention relates to an ink jet printing head; it relates moreparticularly to its applications to industrial plotters.

BACKGROUND OF THE INVENTION

These latter appear in different forms, but all are composed of a partcarrying the sheets (support medium) for receiving the plots, anotherpart supporting one or more styluses, these two parts being set inrelative movement, either by moving the sheets, or by moving thestyluses, or by a combination of both methods.

In these industrial plotters, the styluses depositing the ink are ofteneither ballpoint pens, or felt tips, or else hollow tips for specialinks of Indian ink type.

These styluses have several drawbacks among which may be mentioned theneed of contact, during printing, between the printed medium and thestylus. Now, in the technology of the stylus and the quality of thesurface of the printed medium, the quality of the plot is not constantand is not always the best, particularly when the medium to be printedis abrasive on the surface (paper . . . ), which results in a usefullife of the stylus which is often very short, regardless of itstechnology.

Another drawback resides in the fact that the absence of printing duringmovement of the printed medium requires the stylus to be lifted, whichresults in a considerable waste of time during execution of the plot.Furthermore, on resuming the plot, the ink of the stylus does not alwaysflow instantaneously, whence marks appear at the beginning of the plot.

A difficulty also arises as regards the compatibility between the ink,the technology of the stylus, and the quality of the medium to beprinted which is not obvious and which results in greatly limiting thequality and the useful life of the plot on its medium. Generally, foreach type of medium (different qualities of paper, mylar, polyester film. . . ), it is advisable to use a different type of stylus (ballpointpens, felt tips, hollow tubes . . . ).

OBJECT OF THE INVENTION

The object of the invention is to avoid all of the above mentioneddrawbacks, by proposing a solution of replacing existing styluses by acontinuous ink jet printing head adapted to the needs of industrialplotting.

SUMMARY OF THE INVENTION

The invention concerns more precisely a continuous ink jet printing headassociated with an ink flow system, comprising a recovery channel, thishead being formed essentially of a modulation system, an ejectionnozzle, electrodes for charging the drops, and a deflection system,characterized in that this deflection system includes a set ofdeflection plates placed and fed by means of an electronic circuit sothat the electric field (E) for deflecting the drops is orientablethrough an angle which may vary from 0° to 180°.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following explanationsand the accompanying FIGS. in which:

FIG. 1 illustrates diagrammatically the standard ink jet technique;

FIG. 2 is an illustration of the thicknesses of the variable lines whichmay be obtained by the ink jet technique;

FIGS. 3a and 3b illustrate diagrammatically and respectively a printinghead according to a first embodiment of the invention seen from thefront, and the deflection plates of the head seen from above;

FIGS. 4a, 4b and 4c illustrate the operation of the set of deflectionplates of the first embodiment of the invention;

FIG. 5 shows a plot on a medium with orientations of the patterns ofdifferent dots;

FIG. 6 illustrates the corrected relative head-medium path as a functionof the rotation radius of the pattern of dots;

FIGS. 7a and 7b illustrate respectively in a front view a secondembodiment of a printing head according to the invention and, seen fromthe top, the combination of the set of deflection plates cooperatingwith a recovery channel of adapted shape;

FIG. 8 illustrates the path of the drops in the second embodiment;

FIGS. 9a, 9b, 9c and 9d illustrate the operation of the deflectionplates of the second embodiment;

FIG. 10 illustrates the function played by the adapted recovery channel;

FIG. 11 illustrates a third embodiment of a set of deflection platesaccording to the invention;

FIG. 12 is an example of an electronic circuit for controlling thedeflection voltages;

FIGS. 13 and 14 are diagrams illustrating the parameters for controllingthese voltages.

For the sake of clarity the same elements bear the same referencesthroughout the FIGS.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 is a FIG. of the known art which illustrates the ink jet printingtechnique concerned.

This technique consists of forming a continuous jet of calibrated drops(je) supplied by a modulation system (8) connected to an ink feed device(80) having an ejection nozzle (81). At the level of the break up of thejet leaving the ejection nozzle (81), the drops are chargedelectrostatically by means of charging electrodes (7). Deflection plates(3, 5) creating an electric field deflect them from their path. All ofthese modulation, ejection, charging and deflection means constitute theprinting head (T). If the medium (S) on which it is desired to write andthe printing head (T) are in relative movement, a printing matrix isformed. In the example described, it is an "M". All of the drops notused are recovered in a channel (1) before being recycled in the inkflow system (2).

As shown in FIG. 2, different thicknesses of lines may be obtained byjuxtaposing several drops. Lines of thickness e=0.1, 0.2, 0.3, 0.4, 0.5have been shown. They are composed respectively of a number of drops(nb) varying from 1 to 5 and creating on the medium an impact on theorder of 130 microns in diameter.

It is seen therefore, on the one hand, that between the printing headand the medium there is no bearing point, which eliminates the majordrawbacks connected with this contact, drawbacks that are encountered inmachines where a stylus is used for forming a plot on a support medium.On the other hand, it is seen that a single printing head allows linesof different thicknesses to be obtained, whereas each thickness requiresthe use of an adapted stylus in the case of conventional plottingmachines.

This description therefore shows the advantage of replacing the markingelements generally used in industrial plotting machines by an ink jetprinting head; here it is an obviously advantageous application of theink jet technique in the field of industrial plotting.

This invention also has as an object a new printing head particularlyadapted for this application as will now be described. In theconventional ink jet technique, as is illustrated in FIG. 1, the jetprints columns of dots. In an industrial plotting machine, the supportmedium may move in all directions with respect to the stylus. Now, inthe known technique of the continuously deflected jet, printing columnsof dots (patterns), these latter are always situated in the same plane,which is generally perpendicular to the direction of movement of theobject to be marked (see FIG. 1).

One of the important characteristics of the invention resides thereforein the fact that, through a new arrangement of the deflection plates, anorientable electric field is obtained for deflecting the drops. Underthese conditions, it becomes possible to maintain the column of dots (1to 5 dots in the example of FIG. 2), composed of a set of deflecteddrops (pattern) in a plane which is always perpendicular to the relativedirection of movement of the medium to be printed, whatever thisdirection. An electronic control circuit cooperates with this new set ofdeflection plates according to the invention, which circuit will bedescribed hereafter. In addition, in some embodiments, an adaptation ofthe shape of the channel contributes to the success of the process.

A first embodiment of a printing head according to the invention isshown, in a top view, in FIG. 3a, whereas FIG. 3b illustrates the newset of deflection plates of this head, seen from the top.

There is a modulation body (8) receiving the pressurized ink and havingan ink ejection nozzle forming the ink jet, an electrode for chargingthe drops associated with a detector (6) of the passage of the drops, agutter (1) for recovering the ink drops not used for printing,associated with an ink recovery pipe 2 under a partial vacuum (arrow f).

In accordance with the invention, this printing head comprises acombination of three deflection plates (3, 4, 5) for creating anelectric field for deflecting the charged drops, orientable through anangle which may vary from 0° to 180°. Two of these plates (3) and (5)are parallel to each other, and the third one (4) is situated in a planeperpendicular to the preceding ones. According to another characteristicof the invention, this set of three deflection plates (3, 4, 5)cooperates with a special recovery channel structure (1) allowing theorientation from 0° to 180° of the deflected drops. The drops fall intoa circular orifice (1a) formed in a narrowed extension (1b) of the flatreservoir forming channel (1). This orifice is situated in the axis ofthe head (T).

FIGS. 4a, 4b and 4c illustrate by way of example three drop deflectionangles, namely:

β=0°

β=45°

β=180°.

To each of these deflection angles there corresponds an electric field(E) created by combining the high voltage supply for each of the threeplates.

For an angle β of 0°, only the deflection plate (5) is fed.

For an angle β of 45°, the deflection plates (4) and (5) are fedsimultaneously.

For an angle β of 180°, only the deflection plate (3) is fed.

These examples of oriented deflections are made in FIGS. 4a, 4b and 4cwith a pattern (T) of four drops, the undeflected jet being shown by awhite dot (X) in the gutter (1). By changing the number of drops in thepattern, as explained in FIG. 2, it is then possible to make a plot bychoosing the thickness of the line. This line thickness is kept whateverthe direction of advance of the support medium, by orienting thedeflection field. This is oriented perpendicularly to the direction ofadvance of the support medium with respect to the plotting head.

In FIG. 5 is shown a plot (100) on a support medium (not shown) withorientations of the pattern of different dots, so as to keep a constantline thickness (β=180°, 135°, 90°, 45°, 0°).

With respect to a fixed point on the printing head, depending on theangle of deflection at the time, the pattern of dots do not always fallin the same position. Therefore, it is necessary to make a pathcorrection in the relative movement of the printing head with respect tothe support medium to be printed as a function of the angle retained inthe deflection of the pattern of dots.

In FIG. 6 there has been shown, as a function of the radius of rotation(R) of the pattern of dots around the axis of the channel, the relativecorrected head-support medium path (TR), for a given curve plot (TC).This printing head variant for ink jet plotting requires a program forcorrecting the path of the curve plot taking into account the radius ofdeflection of the pattern of dots used for the plot.

A second embodiment of a printing head of the invention is illustratedin FIGS. 7a and 7b, FIG. 8 and FIGS. 9a, 9b, 9c and 9d. As before, FIG.7a shows diagrammatically the printing head seen from the front and FIG.7b, the set of deflection plates seen from the top with the recoverychannel (1) exhibiting an original shape adapted to this application.The number of deflection plates is here again equal to three. The sameelements as in the preceding embodiment are found here. In thissolution, only the shape of the channel and the way in which the dropsare used for the printing are changed. In fact, the drops not used forprinting are this time systematically deflected into the channel (1)which, in accordance with a characteristic of the invention, comprises asemicircular reception chute (111). The drops used for printing aredeflected so that the center of the pattern of printed dots, whatevertheir number, is in the axis of the head and therefore in the center ofthe semicircular chute (111) of the channel (1). An example is given inFIG. 8 with a pattern of 5 dots, in a figure in which only themodulation system (8) and channel (1) with its semicircular chute (111)appear.

In FIGS. 9a, 9b, 9c and 9d are shown diagrammatically severalorientations of the deflection field, respectively 0°, 45°, 90° and180°; with a pattern of four dots. The drops not used for printing aredeflected and shown by the white spot (X) in the chute (111) of therecovery channel (1).

The advantage of this architecture is the possibility of forming a plot,with programmable line thickness, by acting on the number of drops,while keeping this line thickness whatever the relative path of the headand the printed support medium, without needing a relative correction ofthe path of the head with respect to the support medium as was the casein the preceding embodiment (FIG. 6).

Another advantage of this second embodiment resides in the fact that theprinting head may withstand considerable accelerations because of theparticular arrangement of the recovery channel (1). In fact, during anacceleration (and deceleration) phase of the head in the direction ofthe vector (V₀) shown in FIG. 10 (that is to say, perpendicular to theplane of deflection of the drops), these latter are shifted slightly inthe axis of vector (V₁) (FIG. 10) during their path. Therefore, there isa risk for the drops not intended for printing of no longer being ableto fall into the recovery channel. The arrangement of the channelproposed in the embodiment of FIGS. 7 to 10 mitigates this drawback tothe extent that the arc of the circle formed by the chute (111) offers,in the direction of (V₁), a very large channel dimension with respect tothe other solutions.

The two advantages of this second embodiment (namely, the uselessness ofa path correction and the possibility of withstanding highaccelerations) make it an advantageous solution for applications inwhich the speed of plotting is important.

FIG. 11 illustrates a third embodiment of a printing head according tothe invention. In this configuration, four deflection plates (3), (4),(5), (9) are provided. The deflection plate (9) is parallel to thedeflection plate (4) and perpendicular to the other two (3, 5). Arepresentative case has been shown by way of example in which thedeflection plates (4, 5) are live. The calculation of distances (a) and(b) between the plates will be explained subsequently. For the purposeof holding the printing head in position, the fourth deflection plate(9) is retractable.

In accordance with the invention and as has already been mentioned, acircuit (C) for controlling the high deflection voltages cooperates withan orientable deflection printing head according to the invention.

Therefore, the purpose of the circuit (C) for controlling the highdeflection voltages is to bring the three deflection plates (3), (4),and (5) of FIG. 3 (or the three or four deflection plates (3), (4), (5),(9) in the embodiments of FIGS. 7 and 11) to adequate potentials so thatthe deflection plane of the drops is perpendicular at all times to theaxis of the relative movement of the printed support medium with respectto the printing head. The voltages of the deflection plates arereferenced respectively as (V₃, V₄, V₅, V₉).

In a particular example shown in FIG. 12, the angle β of the relativemovement of the printed support medium with respect to the printing headis transmitted to a control device (200). The control device (200)searches each instant in a memory (30) for the values of the voltages(V₃, V₄, V₅, V₉) which it is necessary to apply to the electrodes so asto obtain the orientation of the pattern along with the desired angle(β). The same control device (200) continuously controls devices (201)for supplying the deflection plates with voltage. These voltage arecharacteristic of a given printing head. An example of calculating thevalues of these voltages is given hereafter.

The voltages of the deflection plates are adjusted so as to create atthe level of the drops a resultant electric field (E), of givenintensity, and oriented in the plane perpendicular to the axis ofrelative movement of the printed support medium with respect to theprinting head.

Calculations of the voltages (V₃, V₄, V₅, V₉) depends on the geometry ofthe head considered and requires, strictly speaking, the resolution ofthe physical problem of the distribution of the electric potential inthe printing head, taking into account the geometrical details of thehead. This resolution may be made by different methods, includingcomputerized digital resolution methods. When the position of the dropsis centered with respect to the electrodes and when the size of theelectrodes is large with respect to that of the pattern, a value can beobtained close to the deflection voltages required for obtaining theelectric field of value (E) oriented along desired angle (β).

In this case the following formulae are used: ##EQU1## the values of(Ex) and (Ey) being calculated from the formulae given hereafter.##EQU2##

In a particular example of application of the embodiment illustrated inFIG. 11, the distances between the electrodes are a=b=5 mm, and thedeflection plate (9) is at a zero volt potential. FIG. 13 gives therelationship required between the values of the voltages (V₃), (V₄), and(V₅), so that the value of the resultant electric field at point (j)situated on the axis of the undeflected jet is: E=0.25 MV/m.

In practice, and by way of illustration, for an angle β=60°, FIG. 14gives (V₅)=750 V and (V₃)=0 V. FIG. 13 then gives (V₄)=1300 V. The valueof the resultant field is, under these conditions, equal to 0.25 MV/m.

Other voltage combinations may be contemplated for obtaining thisresult, these combinations follow from the same equations.

In the example given, the ink ejection nozzle has an internal diameterof 25 microns, and the voltages applied to the charging electrode are onthe order of 150 V at most for obtaining the desired line widths (0.1 to0.4 mm).

As was mentioned above, a privileged application of a continuous jetprinting head and more particularly an orientable printing head withdeflection plates is in the field of industrial plotters. Any contactbetween the marking element and the support medium to be marked isavoided with all the advantages that that entails. In addition, thethickness of the line may be chosen and kept constant whatever the axisof relative movement of the head with respect to the support medium.Finally, even in an acceleration (deceleration) phase, recovery of theunused ink drops is possible through the provision of a recovery channelwith adapted geometry.

I claim:
 1. An ink jet printing head comprising:(a) an ink feed device;(b) a modulation system having an ejection nozzle operatively connectedto said ink feed device; (c) a charging electrode downstream of saidejection nozzle in position to charge ink droplets ejected by saidejection nozzle; (d) a first deflection plate downstream of saidcharging electrode and adjacent the path of the ink droplets; (e) asecond deflection plate downstream of said charging electrode by thesame distance as said first deflection plate, adjacent the path of theink droplets, and perpendicular to said first deflection plate; (f) athird deflection plate downstream of said charging electrode by the samedistance as said first and second deflection plates, adjacent the pathof the ink droplet, parallel to said first deflection plate, andperpendicular to said second deflection plate; and (g) electroniccircuit means for controlling the voltages on said first, second, andthird deflection plates so that the electric field generated by saidfirst, second, and third deflection plates is oriented along an angle βwhich may vary from 0° to 180°.
 2. An ink jet printing head as recitedin claim 1 and further comprising an ink recovery channel placeddownstream of said deflection plates.
 3. An ink jet printing head asrecited in claim 2 wherein said ink recovery channel has a circularorifice centered on the axis of said ejection nozzle.
 4. An ink jetprinting head as recited in claim 2 wherein:(a) said ink recoverychannel comprises a semicircular chute centered on the axis of saidejection nozzle and (b) the center of the pattern of the drops used forprinting is on the axis of said ejection nozzle.
 5. An ink jet printinghead as recited in claim 1 and further comprising a fourth deflectionplate downstream of said charging electrode by the same distance as saidfirst, second, and third deflection plates, adjacent to the path of theink droplets, perpendicular to said first deflection plate, parallel tosaid second deflection plate, and perpendicular to said third deflectionplate.
 6. An ink jet printing head as recited in claim 5 wherein saidfourth deflection plate is retractable.
 7. An ink jet printing head asrecited in claim 1 wherein said electronic circuit means causes theplane of deflection of the ink droplets to be perpendicular at all timesto the axis of relative movement of the media being printed on withrespect to said ejection nozzle.
 8. An ink jet printing head as recitedin claim 1 wherein:(a) adjacent edges of said first, second, and thirdelectrodes are equidistant from each other by a distance of about 5 mmand (b) the value of the electric field generated by said first, second,and third deflection plates is 0.25 MV/m.